To get the petroleum, you'll need to use a series of drilling machines to extract the precious black gold from the marine floor. This is something that's done almost exclusively with the mouse, and while you'll need to watch a few tutorials to understand how it works, it's pretty easy and fun to do.
Either way, extracting the petroleum is only the beginning, since the aim of the game is to sell barrels of the stuff. To play well, you'll want to try to take advantage of the moments in which you can get more money for each barrel. And, as you play, you'll get experience points and improvements that will make your work more fun and complex.
The Oil Blue is a very interesting game. If you love strategy and management games, you should definitely check it out.